Evaluation Metrics

The following metrics were used to evaluate each Slow Street:

• Average Daily Traffic Volume

• Typical Median Daily Vehicle Speed

• Collision History

Characteristically, Slow Streets are similar to other residential street facilities like Bicycle Boulevards (streets with low vehicle traffic volumes and speeds, designated and designed to give bicycle travel priority). Per NACTO guidelines, these facilities typically have two major traffic operation conditions that need to be met in order to be considered low-stress:

• Typical Bicycle Boulevard: Vehicle volumes of 1,500 per day or less

• Typical Bicycle Boulevard: Vehicle speeds of 25 mph or less

A street meeting these baseline conditions constitutes a street that is lower-stress, calmer and appropriate to serve as a major pedestrian or bicycle route.

Low vehicle volumes mean the street does not have many vehicles driving through and is quieter. Put differently, this means that people who walk, roll or or bicycle in the street do not encounter or interact with as many moving vehicles.

It's also important for people to drive vehicles slowly; driving at lower speeds provides more time for vehicles to see or be seen by other road users and to stop or yield to those users. Speed is a key predictor of crash survival. When a person is hit by a vehicle traveling 20 MPH there is a 90% chance of survival; at 40 MPH the survival rate drops to 40%.

The SFMTA, with direction from the Board of Directors, has adopted more stringent targets for ensuring that Slow Streets function as true, low-stress streets:

• Slow Streets Program: Vehicle volumes of 1,000 per day or less

• Slow Streets Program: Vehicle speeds of 15 mph or less

These more stringent targets can be met with the addition of traffic calming and volume management treatments on Slow Streets.

Traffic volumes and speeds

Traffic volumes and speeds

Traffic volume and speed data was collected for 48-hour periods on weekdays between January and April 2023. Vehicle volumes are reported for each data collection location and also as corridor-length averages. Similarly, vehicle speeds are provided for each location along a corridor, and a typical median speed for the entire corridor is also shown.

The Program-wide findings aggregate the data collected on the individual Slow Streets to show overall trends in the performance of Slow Streets. Data collected in 2023 is compared to 2021 data to measure the change in traffic safety conditions two years into the Program. There is also data on 11 of the current Slow Streets from before they were implemented and incorporated into the Program, allowing for a high-level understanding of how the Slow Streets Program has broadly affected speed and volumes. Comparing 2021 data to 2023 data allows for a more detailed look into how individual streets and sections are performing, and helps to prioritize efforts to reduce speeds and volumes.

Collisions

Traffic collision data was analyzed to measure traffic safety on Slow Streets. The collision analysis examined reported collisions involving all modes (vehicle, bicycle, pedestrian, and other mobility device) that occurred on the corridor and within 20 feet of intersections on the corridor.

The Program-wide collision findings aggregate the collision data collected on the individual Slow Streets to show overall trends in the performance of Slow Streets. Consistent with the methods used in the 2021 Slow Streets Evaluation Report, a baseline pre-implementation collision rate was established by using collision data from 2017 up until the date of implementation for each Slow Street and compared to the post-implementation collision rates. In comparison to the 2021 Evaluation Report, which analyzed the frequency of collisions on each Slow Street corridor, this analysis normalized collision rates per corridor, accounting for the length of each Slow Street in relation to the frequency of collisions by reporting on a monthly collision rate per mile of Slow Street.